Ceramic matrix composites (CMCS) are a class of structural materials for service at very high temperatures that have a variety of applications in the aerospace, aircraft propulsion, and power generation industries. Among these applications are rocket nozzles, turbine engine components, and heat exchangers.
Processes for making CMCs start with a preform of high strength fibers that retain their strength at high temperatures (for example carbon, a metal carbide, or a metal oxide). The preform may be prepared by a variety of processes similar to those used in the textile industry that either directly yield useful shapes or produce material, such as woven fabric, that may be subsequently shaped.
The fiber preform is consolidated by emplacing therein a matrix of ceramic material by one of several methods such as chemical vapor infiltration (CVI), gas phase reaction bonding, liquid phase reaction bonding, or slurry infiltration followed by hot pressing.
In the CVI process, illustrated by U.S. Pat. No. 5,079,039 to Heraud et. al., a reactive gas or mixture of gases simultaneously infiltrates the preform and pyrolyzes to form a ceramic matrix. CVI requires sophisticated and expensive equipment and, due to the necessity for operating under conditions where the deposition rate is low, is a slow and inefficient process when used as the sole process to fabricate CMCs.
The gas phase reaction bonding process has been developed primarily for silicon nitride matrix CMCs, where a green body made with silicon powder is nitrided with nitrogen gas. This process is illustrated by U.S. Pat. No. 5,510,303 to Kameda et al., which discloses a CMC material manufactured by forming a matrix containing reaction sintered silicon carbide as the primary component and nitriding the free metal silicon produced in the sintering process to convert it to silicon nitride. Gas phase reaction bonding has been developed only for silicon nitride and silicon carbide matrices.
Liquid phase reaction bonding has been used primarily for the production of silicon carbide matrix composites by infiltrating a preform containing carbon particles with liquid silicon. After reaction, the matrix consists of SiC and usually some free silicon. This process is illustrated by U.S. Pat. No. 5,552,352 to Brun et al., which discloses a composite fabricated from coated reinforcement fibers admixed with a carbonaceous material which is infiltrated with molten silicon. Liquid phase reaction bonding has been developed only for silicon carbide and silicon matrices.
In the slurry infiltration process, typically a tow is passed through a slurry containing matrix material, wound onto a drum, dried, laid up in the desired configuration, and hot pressed, as illustrated by U.S. Pat. No. 5,407,734 to Singh et al. Processes that use slurry infiltration followed by hot pressing require equipment operating at high pressures and temperatures in excess of 1800.degree. C. to 2000.degree. C.
Other methods of preparation involving fluid precursors have been suggested which involve the use of oxygen-free organometallic precursors. In general, however, the precursor materials employed are extremely air and moisture sensitive and require expensive inert environment fabrication.
In addition to the basic techniques for fabricating CMCs as summarized above, various additional processing steps can be integrated into the fabrication process to enhance various properties. The use of such additional steps obviously depend upon the intended use of the final CMC product. One such well-known step is the use of an interface coating to enhance the mechanical properties of a CMC, as exemplified by the teaching in U.S. Pat. No. 4,752,503 to Thebault. Such a coating consists of carbon or boron nitride applied by a CVI process. Other techniques are disclosed in U.S. Pat. No. 5,110,771 to Carpenter et al., U.S. Pat. No. 5,455,106 to Steffier, and U.S. Pat. No. 5,422,319 to Stempin et al.
U.S. Pat. No. 4,576,836 to Colmet et al. discloses a process for making oxide matrix CMCs by vapor phase in-situ hydrolysis of halide vapor, and includes a preliminary rigidization step of repeated liquid phase impregnation with a hydroxide, alkoxide, or organometallic, with intermediate drying/calcination.
One potentially low-cost fabrication route for carbide matrix CMCs is infiltration of a porous carbonaceous preform with a liquid containing a compound of a carbide-forming metal in solution or suspension and then heat treating the infiltrated body to form metal carbide in the interstices of the preform. The application of this method to form two-phase carbon-metal carbide bodies from porous monolithic graphite has been known and practiced for many years, as illustrated by U.S. Pat. No. 3,432,336 to Langrod et al. Other processes directed to the production of metallic carbide composites are described in U.S. Pat. No. 4,576,836, to Colmet et al., U.S. Pat. No. 4,196,230 to Gibson et al, and U.S. Pat. No. 5,759,620 to Wilson et al.
All the fabrication processes and additional steps summarized above require specialized equipment, long processing times, complex processing steps, high processing temperatures, and, in some cases, high processing pressures. They also are applicable with only a limited number of matrix and reinforcement compositions. These and other factors contribute to the high cost of CMCs, which in turn has limited their commercial acceptance.
Therefore, what is needed is a process for fabricating ceramic matrix composites that has advantages of lower cost, greater simplicity, shorter production times, and utilization for a wider range of matrix compounds.